Page 100 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

per alloys, since cuprite allows the oxide film to conduct electronically. It does this by transfer­
ring an electron from a cuprous ion to a cupric ion—in effect, by migration of the positive holes.
Ionic conduction in cuprite occurs, therefore, by migration of the vacant sites in the copper lat­
tice, and oxidation takes place by the migration of copper through the film to the surface. Cor­
rosion processes are facilitated because 0 ~ and Cl" can be transported toward the metal, and
2
Cu + can travel outward. Despite these defects, cuprite often preserves important detail of the
3
original surface of an object by acting as a marker layer (Organ 1963 a,b; Chase 1994).
Some buried bronzes preserve a patina that is principally cuprite, such as the patina on the
bronze head of the Emperor Hadrian from the second century (PLATE 8), now in the collections
of the British Museum. A predominandy cuprite patina under vestigial malachite covers the
bronze casting of the Egyptian god Horus (PLATE 9) from the collections of the Shumei Cultural
Foundation, Japan.
There is often an epitactic relationship between the growth of the cuprite layer and the ori­
entation of the copper-alloy substrate, and this helps to preserve pseudomorphic detail within
the corrosion. The dendritic pattern of cast bronze, for example, may be preserved within the
structure of a cuprite crust as the corrosion replaces the metallic phase. Twinned crystal struc­
tures in worked and annealed objects can also preserve faint traces of the grain boundaries or
twin lines within the corrosion layer.
Cuprite can form on a bronze by exposure to moist air, by tarnishing in use, or during bur­
ial. For the majority of bronzes, cuprite is the corrosion layer within the metallic surface layers
and immediately overlying the original metallic surface. When bronze alloys corrode, the most
common event is the formation of cuprite within the immediate surface layer of the alloy. As the
corrosion process develops and copper ions migrate outward, the cuprite may grow over the
originally developed corrosion and present a number of different layers when examined in cross
section. Some of the cuprite will be seen to have developed below the original surface, often still
preserved within the corrosion, and some will have developed above this original surface,
embedding the marker layer within the cuprite crust.
Cuprite can also occur, however, as a primary component of the metal itself. It can result
from oxygen absorption and the formation of copper-cuprous oxide eutectic, or it can appear as
small globules of cuprite from the original melt. Cuprite may show strong polarization colors
when viewed under crossed polars; this is one method of distinguishing between inclusions of
cuprous oxide and those of cuprous sulfide. The two most common nonmetallic inclusions in
ancient bronze alloys are copper oxides and copper sulfides, which may be distinguished by
polarized light microscopy. The oxide is usually cuprite and shows strong polarization colors
when viewed under crossed polars. With primary cuprite inclusions, a stationary cross may
often be observed. Copper sulfides, such as chalcocite (Cu 2S), however, remain black under
crossed polars and do not reveal any anisotropic characteristics or pleochroism.

O X I D E S AN D H Y D R O X I D E S
83